From: Ian Lance Taylor <ian@airs.com>
To: "Matt Lee" <reachmatt.lee@gmail.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Question about RTL for bitwise AND
Date: Fri, 14 Apr 2006 03:21:00 -0000 [thread overview]
Message-ID: <m364lclu4a.fsf@gossamer.airs.com> (raw)
In-Reply-To: <e94302560604131631r388a187bv1b8da028812eb15b@mail.gmail.com>
"Matt Lee" <reachmatt.lee@gmail.com> writes:
> Here is what I have,
>
> case AND:
> case IOR:
> case XOR:
> *total = COSTS_N_INSNS (1);
> return true;
> case ASHIFT:
> case ASHIFTRT:
> case LSHIFTRT:
> if (GET_CODE (XEXP (x, 1)) == CONST_INT)
> *total = COSTS_N_INSNS (INTVAL (XEXP (x, 1)));
> return true;
>
>
> This looks quite OK to me. I tried debugging if rtx_costs was doing
> something wrong. I do see rtx_cost being invoked for the lshiftrt
> expressions in question, but never for the "and". Seems like GCC had
> pre-decided to only use lshiftrt even though it is expensive.
Your computations look wrong to me. You are saying that an AND always
costs 1 instruction. But you need to actually look at the operands.
Specifically, the compiler is going to compare these two:
(and (reg ...) (const_int ...)
(and (ashiftrt (reg ...) (const_int ...)) (const_int 1))
> Any other ideas? Btw, I couldn't find prefer_and_bit_test in dojump.c.
I'm looking at the current version of gcc. prefer_and_bit_test was
added 2004-03-20, so I guess that just missed 3.4. I don't have 3.4
sources handy. In general the decision is made in the do_jump
function.
Ian
next prev parent reply other threads:[~2006-04-14 3:21 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-13 16:57 Matt Lee
2006-04-13 20:29 ` Ian Lance Taylor
2006-04-13 23:31 ` Matt Lee
2006-04-14 3:21 ` Ian Lance Taylor [this message]
2006-04-14 21:49 ` Matt Lee
2006-04-18 4:33 ` Ian Lance Taylor
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